The present invention relates to an improved nickel base brazing alloy, particularly suitable for diffusion brazing superalloys, including gamma prime superalloys and a method of nonpressure diffusion brazing using the improved brazing alloy of this invention.
Diffusion brazing relies upon solid-state diffusion or movement of metal atoms across the interface of the brazed joint between the brazing alloy and the base metals. It necessarily follows that diffusion brazing alloys are formulated to complement the chemistry of the parts to be joined. Diffusion brazing allows are thus generally nickel, iron or cobalt base alloys, depending upon the composition of the parts to be joined. High strength superalloys have presented a particular problem for diffusion brazing because of the limited wettability. The problem is to formulate a diffusion brazing alloy which complements the base metals having the requisite properties including microstructure and which may be brazed at temperatures low enough for commercial application. In general, high brazing temperatures adversely affect the properties of the brazed joint.
Reference is made herein to my copending application for U.S. patent filed Dec. 1, 1982, Ser. No. 445,818, now U.S. Pat. No. 4,507,264, which discloses a nickel base brazing alloy including chromium, tantalum, boron, aluminum and a rare earth, preferably yttrium or lanthanum. The improved brazing alloy of this invention has similar concentration of chromium but differs in other material respects and permits lower temperature brazing and improved wetting. In addition to the prior art patents cited in my above referenced copending application, the prior art includes commercial diffusion brazing alloys having greater concentrations of chromium, without iron. These commercial alloys do not, however, exhibit the improved microstructure of the brazing alloy of the present invention, as described more fully hereinbelow. Further, the elimination of silicon eliminates the precipitation of secondary phases of silicon which may result in hard spots and lack of uniformity in the microstructure of the brazed joint.
The applicant also previously developed a nickel base commercial brazing alloy including similar concentrations of chromium and iron, with extra low carbon, but with a higher level of boron and also including silicon for improved braze wetting, as described below. This alloy is not, however, suitable for many diffusion brazing applications, especially thin sections, where joint ductility and strength are critical, nor on wide gap conditions where the silicon does not go into solid solution. Further, the brazing temperature of this alloy is relatively high, about 2100.degree. F. This alloy also has a very wide spread between the solidus and liquidus temperatures, making the alloy more sensitive and difficult to control when "superheating" above 2100.degree. F. is utilized, e.g., diffusion-brazing of MM-002.degree. blade components at 2175.degree. to 2225.degree. F., the alloy's primary secondary solution treatment temperature. It was, therefore, determined that a more sluggish flowing, silicon-free braze alloys would be required for use on superalloys at these higher braze temperatures.
Conventional diffusion brazing alloys require a gap in the parts to be joined of less than 0.002 inches to get a good or acceptable microstructure. This limitation handicaps and limits joint design freedom and braze flow, requiring preplaced braze foils or tapes. In fact, comemrcial applications in the aerospace industry have gaps of 0.002 to 0.006 inches which results in a poor microstructure with conventional brazing alloys. Further, microhardness surveys of the brazed joint with conventional diffusion brazing alloys exhibit coarse, brittle "Chinese script" borides as secondary phases and often the gaps are not filled. To obtain an acceptable braze, it is necessary to use a long brazing cycle at excessive temperatures or a post braze soak of as much as 24 hours, or more. Further, a pressure fixture is recommended to obtain an optimum microstructure. The brazing alloy of the present invention eliminates the requirements for a pressure fixture and is capable of brazing gaps of 0.020 inches or more, without adversely affecting the joint microstructure.